Methods of controlling and optimizing processes for producing chemical compounds are well known. The control of parameters such as temperature, pressure, mixing conditions, relative volumes of reactants, and uses of catalysts are generally well understood. Traditionally, newly discovered chemical compounds and processes involving either the production of such compounds, or processes involving the use of such compounds, have initially been carried out by researchers in “bench scale” environments. Particularly promising chemicals or processes may ultimately be produced in quantity by application to industrial-scale processes. Often, problems are encountered in scaling up a process from the laboratory to industrial-scale production.
Problems associated with moving from bench scale production to industrial-scale production often involve changes in process conditions between the bench scale environment and the industrial environment. For example, the temperature of the reactants in a small beaker or flask in a laboratory is much easier to keep constant than the temperature in a production tank having a capacity of hundreds of liters, as is often the case in a chemical processing plant. Variations in other process conditions within a large tank are also more difficult to control, and frequently affect the quality and yield of the desired product.
An alternative to building large scale chemical reactors is to operate a plurality of relatively smaller reactors in parallel. This can minimize the problem inherent in scaling up from the bench scale environment to the industrial environment, particularly if the plurality of chemical reactors operating in parallel are similar to the reactor employed in bench scale development.
Recently, much attention has been directed to the use of micro scale reactors for development of both chemical processes and chemicals. These types of reactors offer several advantages. As noted above, the control of chemical processes within very small reactors is much simpler than control in a large-scale production tank. Once a reaction process has been developed and optimized in a micro scale reactor, it can be scaled up to industrial production level by replicating the micro scale reactors in sufficient quantity to achieve the required production output of the process. If such reactors can be fabricated in quantity, and for a modest cost, industrial quantities of a desired product can be manufactured with a capital expenditure equal to or even less than that of a traditional chemical production facility. An additional benefit is that because the volume of material in each individual reactor is small, the effect of an explosion or fire is minimized, and with proper design, an accident in one reactor can be prevented from propagating to other reactors.
A distinction can be made between chemical production systems that are operated continuously, and those operated discontinuously. Discontinuous processing is often referred to as batch processing. As used herein and in the claims that follow, the term “continuous processing” refers to a processing environment in which a continuous stream of material is processed without interruption, generally over a relatively long period, measured, for example, in terms of days or weeks. The continuous process is generally interrupted only for maintaining the processing equipment, and not because the supply of material being treated or consumed has been exhausted. In contrast, the term “batch processing” as used herein refers to a processing environment in which a finite volume of material is processed without interruption, but only until the supply of material is exhausted, in a period that is relatively short, and generally measurable in terms of minutes or hours. Batch processing, rather than continuous flow processing, is advantageous when a limited volume of material is to be processed. While microreactors are advantageous for the reasons noted above, microreactors are prone to fouling. The fluid channels within microreactors are quite small (micro in size, hence the term microreactors), and their small size make them susceptible to being plugged by reactants, product, byproducts, impurities and the like. Thus, most microreactor based chemical production systems are batch processing systems, as microreactors need for frequent maintenance makes them less well suited for continuous processing applications. It would be desirable to provide a continuous processing chemical production system that offers the advantages of microreactors, but does not need to be frequently shut down for maintenance operations.
Many different types of chemical production systems are known. Often in large-scale chemical or pharmaceutical production, reactors are made as large as possible to generate as much volume of product as possible. At the opposite end of the scale, there is often a need, particularly in the development of new compounds or in clinical diagnoses, to produce large combinatorial libraries, with relatively small amounts of different compounds being generated. Chemical production systems optimized for such requirements generally minimize the size of each reactor, and employ different reaction conditions in each reaction chamber, to produce many different compounds in parallel. Exemplary of such systems are patents and patent applications assigned to Symyx Technologies, including: U.S. patent application Publication No. 2002/0045265; U.S. patent application Publication No. 2002/0042140; International Patent Application Publication No. WO 01/93998; International Patent Application Publication No. WO 01/66245; International Patent Application Publication No. WO 00/51720; and U.S. Pat. No. 6,149,882 (collectively referred to as the Symyx references). The Symyx references generally disclose parallel reactors and fluid control systems configured to enable different flow rates to be achieved in different reactors, or using fluid channels with variable flow resistance, which facilitates producing different products in parallel.
In addition to the variable flow resistance described in the Symyx references, other fluid control configurations are known. U.S. patent application Publication No. 2002/0080563 (Pence et al.) discloses a fluid control device for thermal management that includes both a concentric flow, and bifurcated flow. International Patent Application Publication No. WO 01/68257 (Jury et al.) discloses a parallel micro reactor based chemical production system that incorporates bifurcated fluid flow channels.
When a plurality of chemical reactors are operated in parallel, regardless of whether the reactors are micro scale reactors or larger reactors, it should be apparent that raw materials and heating transfer media can be supplied through individual fluid lines to each reactor. While effective, such a configuration requires multiple pumps and too many fluid lines, making fluid control for such a parallel production system much more complicated than required in a single reactor system of larger scale. It would therefore be desirable to provide a more efficient and less costly fluid control system for reactors coupled in parallel. It would be desirable, particularly with respect to systems that include microreactors, to provide fluid control systems that ensure that the volumes and flow rates of reactants and heat transfer media into and out of each reactor are equivalent, so that the process conditions in each reactor are the same, thereby ensuring that the product generated by each reactor in the parallel production system is consistent.
Because parallel reactor chemical production systems are well suited to the production of commercial quantities of a desired product, it is likely that such a system will be operated continuously over long periods of time. Chemical production systems that are shut down for maintenance are not producing revenue; thus, it would be desirable to provide a parallel reactor chemical production system configured for continuous production over relatively long periods of time, such that the system is offline for minimal periods of time.
Several different microreactor designs have been developed for use in a parallel chemical production system. In addition to the micro reactor designs described in the Symyx references, microreactors suitable for such use are also described in U.S. Pat. No. 5,534,328 (Ashmead et al.); U.S. Pat. No. 5,690,763 (Ashmead et al.); U.S. Pat. No. 5,580,523 (Bard); and U.S. Pat. No. 5,961,932 (Ghosh et al.).
The two Ashmead patents describe reactors fabricated from a plurality of interconnected layers. Generally, each layer has at least one channel or groove formed in it, and most include orifices that serve to connect one layer in fluid communication with another. These layers are preferably made from silicon wafers, because silicon is relatively inert to the chemicals that may be processed in the reactor, and because the techniques required to mass produce silicon wafers by etching the required channels and other features into their surfaces are well known.
A disadvantage of the reactors described by Ashmead stems from the rather expensive and complicated process required for manufacturing the devices. While silicon wafer technology has advanced to the state that wafers having desired surface features can readily be mass produced, the equipment required is capital intensive, and unless unit production is extremely high, the substantial costs are difficult to offset. The specific surface features taught by Ashmead require significant manufacturing steps to fabricate. For instance, while forming an opening through a material is relatively easy, forming a groove or channel that penetrates only part way through the material comprising a layer is more difficult, as the manufacturing process must not only control the size of the surface feature, but the depth, as well.
Bard similarly discloses the use of silicon wafer technology to etch channels and/or other features into the surface of a silicon wafer to be used as a micro reactor. Other disclosed fabrication techniques include injection molding, casting, and micromachining of metals and semiconductor substrates. Again, the processing required to fabricate the individual modules goes beyond merely forming a plurality of openings into each component.
The Ghosh patent describes the desirability of sizing fluid channels in microreactors appropriately to provide for laminar flow and mixing via diffusion, rather than mixing via turbulence. Ghosh describes fabricating reactor layers from “green” or uncured ceramic, which once shaped as desired, must be sintered. Significantly, the sintering process changes the size of the ceramic layer, so that the sizes of the features formed into the ceramic layer in the initial stages of production are different than in the finished product.
It would be desirable to provide a reactor design in which the dimensions of the individual components can be rigidly controlled during fabrication, and are not subject to shrinkage, which can negatively effect the dimensions of the finished reactor. This object is particularly important when a reactor design focuses on achieving a laminar flow, because precise dimensional control of fluid pathways in the reactor must be maintained to achieve a consistent laminar flow, precisely controlled pressure drops, and precisely controlled fluidic resistance.
In all of these prior art reactors, relatively complicated manufacturing techniques are required. The manufacture of layers of silicon material requires a large capital investment. Sintering of a ceramic material requires the precise control of the shrinkage process, or individual components of a desired size cannot be achieved. In all cases, the prior art teaches that complicated structures (for example, fluid channels and reaction channels) must be etched or otherwise fabricated in each layer. Additionally, orifices or passages also need to be formed in each layer, so that fluids can move between adjacent layers of the reactor. Thus, a series of different manufacturing steps typically must be performed for each layer. It would be desirable to provide a reactor design offering the advantages described above, but which is relatively simple to manufacture, so as to minimize capital investment in scaling up production from the laboratory to industrial production levels.
As indicated above, while a single micro reactor can produce only a limited volume of product, additional microreactors can be added in parallel to increase production capacity. When additional modular micro reactor units are added, additional systems for reactant supply, heat transfer media supply, and product collection are typically required, which not only increases the complexity of the system, but also requires more space for duplicative parallel fluid systems. Furthermore, even minor differences in feed rates for some of the parallel reactor modules can negatively effect product quality. Finally, more sophisticated control and monitoring are required to manage additional reaction modules and feed systems. It would therefore be desirable to provide a micro reactor capable of n-fold parallelization without requiring that additional fluid and control systems be provided.
Consider an array of identical fluid channels having a single common reactant distribution channel and a single common product collection channel, with the reactant inlet and the product outlet located at opposite ends. If the common reactant distribution and the common product collection channel have the same cross sectional area and if the viscosity of the product relative to the reactants is substantially the same, then the pressure drop through the array can be considered the same. In addition, the resulting flow distribution is fairly even, with only slightly lower flow rates in the central fluid channels.
However, the flow distribution through this array will not be even if the viscosity of the product is significantly different than the viscosities of the reactants. When such an array is employed to process a reaction whose product has a significantly different viscosity compared to the viscosity of the mixture of the unreacted reactants, broad residence time distributions occur in the array due to the fact that the pressure drop in the common reactant distribution channel no longer balances with the pressure drop in the common product collection channel. The flow rates within each individual fluid channel in the array are no longer identical. If the viscosity of the product is significantly greater than the viscosity of the mixed but unreacted reactants, then the flow rates in the individual fluid channels in the array tend to increase across the array for channels closest to the common product outlet. Thus, the highest flow rate is experienced in the fluid channel in the array that is closest to the common product outlet, while the lowest flow rate is experienced in the fluid channel in the array that is located furthest from the common product outlet. This phenomenon is different if the viscosity of the product is less than the viscosity of the mixed but unreacted reactants. For lower viscosity products, the highest flow rate is experienced in the fluid channel in the array that is closest to the common reactant inlet, while the lowest flow rate is experienced in the fluid channel in the array that is located furthest from the common reactant inlet. The greater the relative change in viscosity, the greater the variation in flow rates across the array.
This imbalance leads to different residence times being associated with different fluid channels, resulting in an undesirable imbalanced residence time distribution within the whole reaction unit. In certain cases, the additional residence time can lead to undesired cross reactions, and even clogging of the “slowest” fluid channels. It would be desirable to provide a micro reactor including a plurality of fluid channels that is capable of processing reactant mixtures undergoing a significant viscosity change without the above-described imbalanced residence time distributions and related problems, so that such a micro reactor can be incorporated into a parallel chemical production system.
It should further be noted that for the imbalanced residence time distributions discussed above, only one type of undesirable residence time distribution is discussed relative to reactant mixtures produced in fluid channels in which a plurality of different reactants are mixed. Similar distribution problems can also arise in fluid channels used to direct reactants before mixing, as well as products for collection. It would be desirable to provide a micro reactor that includes a plurality of fluid channels adapted to provide substantially equal residence time distributions for fluid flow within the micro reactor, to provide a micro reactor that can be incorporated into a parallel chemical production system.
Other desirable features for a parallel chemical production system include ease of manufacture, a relatively small footprint, the ability to employ efficient diffusion mixing using a precisely controlled laminar flow, the ability to facilitate n-fold parallelization without requiring additional fluid supply, removal, and control systems, individual reactors that can process reaction mixtures to form a product with a significantly different viscosity, and systems that can provide substantially equal residence time distributions for fluid flow within each different reactor. Currently, the prior art does not include reactors that can achieve these objectives.